This invention relates to a process for the enantioselective hydrogenation of derivatives of ketoisophorone, of the formula ##STR3## in the presence of a cationic rhodium complex.
It is known that compounds of formula I, which can be converted by hydrolysis into phorenol (1) and in a further reaction step into optically active actinol (2), are important intermediates for the manufacture of 3-hydroxy-carotenoids, especially zeaxanthin (Helv. Chim. Acta 63 (1980), 1451). Diverse attempts have hitherto been made to directly enantioselective hydrogenate ketoisophorone (3) used as the educt for the synthesis of actinol (2). In these, the diketone (6) was obtained as the main product not only with ruthenium but also with rhodium catalysts, the optical yields were, however, only low. Attempts have also been made to produce a more suitable substrate for the hydrogenation by derivatizing ketoisophorone (3). Thus, for example, the asymmetric hydrogenation of the cyclic acetal (5) and subsequent hydrolysis gave the diketone (6) with only 10% enantiomeric excess (ee), while in the asymmetric hydrogenation of the methyl enol ether (4) the diketone (6) was obtained with 50% ee (Brunner et al. J. Organomet. Chem. 456 (1993), 71 and Literature cited therein). ##STR4##
The present invention now provides an industrially access to s actinol (2) by asymmetric hydrogenation. It has been found that the enantioselective hydrogenation of acyl derivatives of ketoisophorone in the presence of a cationic rhodium complex with chiral diphosphine ligands in suitable solvents leads to the corresponding acyl derivatives of phorenol in high optical yields which further can be converted to actinol ( 2 ).